Motor selection for large HP
Motor selection for large HP
(OP)
I am considering using medium voltage 2300 volts for several 500 hp compressors now at 600 volts. Is 2300 volt systems more efficeint when considering new equipment.
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Motor selection for large HP
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RE: Motor selection for large HP
I'd suggest doing a search on this forum - there have been several discussions regarding low voltage versus medium-voltage motors.
RE: Motor selection for large HP
RE: Motor selection for large HP
A major consideration in addition to the control equipment costs however might also be serviceability. Make sure your plant electricians are qualified to work on medium voltage. If they are, no problem. But if not, the training can be costly plus they will want more money from then on. Been there, done that, still have the scars from the bean counters!
RE: Motor selection for large HP
Motors will cost more at 4.16 kV. Control gear costs are about the same at 2300V or 4.16 kV.
For 500 HP motors, the savings in going from 600V to 4.16 kV will be in reduced copper. If your runs are short, there may be no savings. Where it has paid out for us is if we have long runs underground. The higher voltages save on wire, conduit and labor costs.
RE: Motor selection for large HP
This analysis is also going to depend a lot on how many "several" is. If you have ten 500 hp motors, that's quite a bit different than if you have three or four.
RE: Motor selection for large HP
In this application there are more than 15, 500 HP motors used for refrigeration. These are currently installed at 600 volts. However for future installation, particularily for motors operated over longer periods of time and some distance away, I was thinking about medium voltage. Any experience on this?
How does the cost of the motor starters/controllers compare? It seems to me that the smaller currents involved at 4160 V should offset the higher costs for insulation.
Regards
RE: Motor selection for large HP
RE: Motor selection for large HP
You also will need to decide on what level of protection you want. Whereas with LV gear you just have to decide on breakers or fuses as SCPDs, in MV it isn't as simple. Here is a grossly simplistic primmer on what you are in for:
MV Circuit Breakers are typically only available in "draw-out" format, called Metal Clad switchgear, where each controller has a separate cubical that can be withdrawn and isolated from the bus bars when it's necessary to work on the equipment. The individual metal enclosed compartments also serve to contain any fault energy to a limited area thus reducing the likelihood of collateral damage. Most people consider this to be the safest way to use MV equipment. But Metal Clad gear is extremely expensive and very large. A basic single starter, regardless of how small, is likely to start at US$80,000. the cost per unit goes down a bit if you have multiple starters in one location.
An alternative is to use Fused Load Interrupter Switch (FLIS) style motor starters. They are less expensive, typically under US$15,000 to start with for an Across-the-Line starter. The drawback to them is that they use fuses, which cannot be "reset" after a fault and require having spares on hand (a very old argument), plus they don't utilize a draw-out mechanism to provide the isolation; they rely upon an air gap switch. Some people do not consider this to be as safe as draw-out when dealing with MV equipment.
The lowest cost MV control gear is now a hybrid of sorts; Fused motor controllers that rely upon a vacuum contactor to interrupt a load, backed up by fuses as SCPDs, but with a switch that is NOT rated to interrupt power if opened under load, especially if it is a fault load. The switch is often on a truck assembly that can be racked out for service isolation however. The big drawback to that style is in the fact that IF someone were to panic and try to open the disconnect under load, it just might blow up on them (obviously I'm not a fan of that design).
So you really have a lot more homework to do before making that decision. Going to MV means making a leap of technology, not just deciding on what will be more efficient.
By the way, do you typically buy your own transformers or do you expect the utility to do that for you? If you are expecting the utility to supply MV to you at your terminals, check with them before deciding anything. Some utilities will only allow certain voltages in certain areas, usually because of what they have available as spares etc. for ongoing service.
RE: Motor selection for large HP
RE: Motor selection for large HP
I think I can conclude that upgrading to medium voltage even for a number of drives may be a long payback. A new plant however or a substantial expansion project may be an opportunity to release some of the low voltage system's capacity and for those loads with the most use through out the year, a medium voltage substation, MCC and drive system may be considered.
Regards,
Mike
RE: Motor selection for large HP